gtk-doc: Fix B verses S issues

Fic the class to handle recipes which set B to somewhere other than S.

(From OE-Core rev: 9d88baea4f3668612c76201c306ec6efe72c281e)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/gtk-doc.bbclass

@@ -7,7 +7,7 @@
 # packages up any existing documentation (so from tarball builds).
 
 # The documentation directory, where the infrastructure will be copied.
-GTKDOC_DOCDIR ?= "${S}"
+GTKDOC_DOCDIR ?= "${B}"
 
 DEPENDS_append = " gtk-doc-stub-native"
 
@@ -18,5 +18,5 @@ EXTRA_OECONF_append = "\
 "
 
 do_configure_prepend () {
-	gtkdocize --docdir ${GTKDOC_DOCDIR}
+	( cd ${S}; gtkdocize --docdir ${GTKDOC_DOCDIR} )
 }